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Old 16-05-2018, 08:17   #61
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Re: Achilles vs Highfield rib

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Originally Posted by travellerw View Post
Everything that holds air is somewhat permeable. There is nothing that will remain at a constant pressure forever, some of the gasses will always pass through the material.

Even tires require filling every so often. This is why some tire shops stock straight nitrogen gas. Nitrogen is a larger molecule. This reduces its ability to permeate materials (however, it still does).
You win a cookie, sir.

Gas permeability is measured through what is called a "zeppelin test" which measures permeability through loss of helium (very small molecules).

PVC tests better under zeppelin tests than CSM/CR does. CSM is actually very porous, which is the point of the CR layer. CR is not UV resistant, but it is very "gas tight".

The glue though is the weak spot. I usually can find a handful of creamer type leaks in all CSM/CR boats. They are usually "self-healing" after the boat depressurizes slightly.

I use a pressure gauge, which measures pressure via tenths of a millibar to 0.25% accuracy at room temperature. I use it for liferafts too.

In all CSM/CR boats, I can usually "find" 1.5 to 3.0% of loss at operating pressure, although they usually "self-heal" after 1 or 2 days of pressure loss. This is why it is so vital to check boat inflation at least once daily during operation.
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Old 17-05-2018, 02:27   #62
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Re: Achilles vs Highfield rib

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Even tires require filling every so often. This is why some tire shops stock straight nitrogen gas. Nitrogen is a larger molecule. This reduces its ability to permeate materials (however, it still does).
Actually the point of nitrogen is just to ensure dry gas. It's the water in normal compressed air that causes the changes in pressure with heat that racers dislike.
(air is 78% Nitrogen, 20 % oxygen, molecules of which are about the same size. Losing the 0.00x% hydrogen or Helium etc is of no consequence)

You can get the same effect using completely dry air (like from a scuba tank) - it's just harder for tyre shops to charge you for 'special' air, so they use nitrogen as it's about the same cost to do.

As a tech diver I had helium in bulk, so used to make a big deal about filling my tyres with helium at the race track to spook other drivers. Was a dumb idea, but fun.

Mike
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Old 17-05-2018, 03:02   #63
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Re: Achilles vs Highfield rib

People's consumer experiences vary greatly, and in addition to the product's quality, are influenced by expectations, personalities, usage, willingness to read manuals, etc. Look at the reviews of any Amazon product, and you'll see people who've enjoyed it for years with no issues, and others who say it broke the first day in service; so it is with things marine. I find that anecdotal reports are most useful when the vast majority of them agree, for good or bad; and I tend to disregard the glowing or damning reports of one person.
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Old 18-05-2018, 11:46   #64
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Achilles vs Highfield rib

The nitrogen in tires is a way to fleece your wallet, completely useless.
It is used often in aircraft Oleo struts and the like, but only because we are after zero water vapor to prevent corrosion.
Racers used to carry Nitrogen gas bottles to the race track to run their air tools etc., cause Nitrogen is essentially a byproduct of the production of oxygen and as such is cheap, real cheap. Used to be thrown away, and Iím sure still is to a large extent.
Since you have large tank of compressed inert gas, what did you put into your tires? The Nitrogen of course.

My favorite is Shell who advertises Nitrogen in their gasoline.
Why would anyone want Nitrogen in their gasoline?
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Old 18-05-2018, 11:59   #65
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Re: Achilles vs Highfield rib

I believe the tires run a bit cooler with nitrogen which can lead to less wear on the tires and better fuel economy
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Old 18-05-2018, 15:49   #66
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Re: Achilles vs Highfield rib

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I believe the tires run a bit cooler with nitrogen which can lead to less wear on the tires and better fuel economy


So says the sales literature.
But then bumping pressure 1 PSI does the same?
N2 has some advantages, without O2 and moisture, you have no corrosion. Irrelevant in tires, but maybe worth it in an aircraft Oleo strut or other applications, maybe. UH-60 main rotor blades are pressurized with N2 and have pressure indicators looking for a cracked main spar for example.
But the tire guys try everything, different expansion rates and all other nonsense to sell you on that $5 extra per tire and Cool looking green valve caps.
If you want to, or course go for it. Doesnít hurt anything.
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Old 18-05-2018, 16:40   #67
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Re: Achilles vs Highfield rib

Well the atmosphere is 80% nitrogen from the get go. It's the water in regular air that causes problems.
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Old 19-05-2018, 01:00   #68
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Re: Achilles vs Highfield rib

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Well the atmosphere is 80% nitrogen from the get go. It's the water in regular air that causes problems.
Yeah... thats what I said :-)

In my little lotus i run 18 psi cold, at 25 psi hot on radial tyres I lose seconds a lap. Dry gas, whether air, nitrogen or even helium :-) makes pressure more controllable.

I use normal air on the road.

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Old 19-05-2018, 06:09   #69
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Re: Achilles vs Highfield rib

While I do not doubt you, that is incredibly low pressure for road racing.
I assume itís a very light car? Which Lotus?
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Old 19-05-2018, 16:20   #70
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Re: Achilles vs Highfield rib

Lotus 7. 540 kilos.
With modern tyres, it takes a bit of work to get heat into them. Sometimes we do pump them up and do sideways, but I had an embarrassing spin during the prolog of Targa Tas doing that.
Some folks go down to 15 psi, but i get nervous about rolling off the rims.

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